1. Field of the Invention
The present invention relates in general to throttle valve devices of an internal combustion engine, and more particularly to throttle valve devices of a type which is equipped with a biasing mechanism for biasing and holding a throttle valve toward and at a predetermined intermediate open position.
2. Description of the Prior Art
In automotive internal combustion engines having a throttle valve actuated by an electric motor, there has been known a biasing mechanism by which, upon failure of the motor, the throttle valve is automatically shifted to a predetermined intermediate open position. One of such biasing mechanisms is shown in Japanese Patent First Provisional Publication 2-500677.
In general, such a biasing mechanism uses both a return spring for biasing the throttle valve in a closing direction and a counter spring for biasing the valve in an opposite, viz., opening direction.
However, due to its inherent construction, the biasing mechanism is poor in freely adjusting a biasing force applied to the throttle valve. Moreover, usage of the two springs tends to increase the cost of the mechanism and thus that of entire of the throttle valve device. Furthermore, usage of the two springs tends to make the mechanism bulky in size and cause the assembly of the mechanism to be troublesome.